please review summary of what we agreed to about derivative

I wrote up a summary from today's meeting in the derivative issue
<https://github.com/w3c/mathml/issues/473#issuecomment-1856752698>. Please
take a look. If you see a mistake, update the comment so that there is one
place that summarizes the resolution. If you think something needs
discussion rather than a tweak/change, then go ahead and add a new comment.

I hope to close the issue later on Friday, so please look right after
reading this.

    Neil